It depends on what pricing and availability is like when you go to build. B550 boards shouldn't be out for at least a couple weeks. But since they are coming soon, supplies of B450 boards are quite sparse at the moment, at least in the US. Those models that are both in-stock and guaranteed to have out-of-box compatibility with Ryzen 3000 processors tend to be priced similar to how the lower-end B550 boards will be priced at launch, so you wouldn't likely be saving much by going with B450, unless supplies stabilize and prices come back down after the launch of B550.
At this time, the 3300X isn't really in stock either though. What other hardware were you planning to put in the system? Was this for gaming, or something else?